During operation of a tapered roller bearing, oil passing through the bearing is stirred by e.g., the rolling tapered rollers, and the stirring resistance of oil tends to increase rotation torque of the bearing. The crossbars of the retainer have guide surfaces that are circumferentially brought into contact with the tapered rollers in the pockets. This increases the shear torque (or shear resistance) of oil between the tapered rollers and the guide surfaces. The increase in such shear resistance also tends to increase rotation torque of the bearing. Trials have heretofore been made to reduce rotation torque of the bearing by modifying the shape of the retainer.
For example, in order to reduce the shear resistance of oil by reducing the amount of oil flowing into the bearing, one conventional retainer is shaped such that a smaller radial gap is defined between the radially inner surface of the retainer and the small flange of the inner race. Some other conventional retainers have recesses in the shape of cutouts formed in the annular portion of the retainer on its small-diameter side to allow oil that has flowed through a gap between the retainer and the inner race into the space between the retainer and the inner race to quickly flow into the space between the retainer and the outer race, thereby reducing the stirring resistance of oil. Still another conventional retainer has recessed surfaces in the form of cutouts formed in the crossbars between the respective guide surfaces and first axial ends of the crossbars and between the respective guide surfaces and second opposite axial ends of the crossbars, to reduce the shear resistance of oil between the tapered rollers and the crossbars (see Japanese Patent 4949652).
Yet another conventional retainer has crossbars formed with guide surfaces each of which is tapered, and has a length of not less than 5% and not more than 20% of the average diameter of the corresponding tapered roller, in a plane perpendicular to the center axis of the tapered roller, to reduce the oil film forming area, and thus the shear resistance of oil between the tapered rollers and the crossbars (see Japanese Patent Publication 2007-24170A).
In another conventional arrangement, in order to reduce stress concentration at the ends of the rolling surfaces of the tapered rollers to withstand further heavy loads, the rolling surfaces have crowning. Especially the combination of tapered rollers having logarithmic large crowning, and raceways having straight or convex crowning is advantageous in bearing function and processing cost. In an arrangement in which logarithmic crowning is used, each tapered roller may have a rolling surface central portion formed at the central portion of the entire length of the tapered roller, and having a straight generating line; and crowning portions each extending from one end of the rolling surface central portion such that its diameter gradually decreases in the direction away from the one end of the rolling surface central portion. In this arrangement, since the rolling surface central portion having a straight generating line is used as a reference in measuring the logarithmic crowning portions, stable quality control is possible (see Japanese Patent 5334665).
If a retainer having the recessed surfaces disclosed in Japanese Patent 4949652 or the guide surfaces disclosed in Japanese Patent Publication 2007-24170A is combined with tapered rollers disclosed in Japanese Patent 5334665, the guide surfaces which are straight in the longitudinal direction of the crossbars will circumferentially contact the rolling surface central portions having straight generating lines and formed at the central portions of the entire length of the respective tapered rollers. Therefore, the contact lengths between the guide surfaces and the tapered rollers are large compared with bearings including tapered rollers having general crowning with circular arc-shaped generating lines. This makes it difficult to effectively reduce the bearing rotation torque.